1. Field of the Invention
The present invention relates to an ink jet recording apparatus in which a liquid such as ink or the like is discharged onto a recording medium in an ink jet recording system.
2. Description of the Related Art
A recording apparatus having the functions as a printer, a copying machine, a facsimile machine, etc., a composite electronic apparatus comprising a computer, a word processor, or the like, or a recording apparatus used as an output apparatus for a work station is constructed to record an image (including characters, symbols, and the like) on a recording medium (referred to as "recording paper" hereinafter) such as paper, cloth, a plastic sheet, an OHP sheet, or the like. These recording apparatuses can be divided into an ink jet type, a wire dot type, a thermal type, a laser beam type, etc. according to their recording systems.
In a serial type recording apparatus in which in recording, scanning is performed in the direction perpendicular to the transfer direction (the paper feed direction or sub-scanning direction) of the recording medium, an image is recorded (main scanning) by a discharge element (a recording head) which is moved along the recording medium, paper is fed (pitch transfer as sub-scanning) by a predetermined amount after completion of recording for one line, and then an image of a next line is recorded (main scanning) on the recording medium which is stopped. This operation is repeated to record the image on the desired range of the recording medium. On the other hand, in a line type recording apparatus in which recording is carried out only by sub-scanning in the transfer direction of the is recording medium, the recording medium is set at the predetermined recording position, and the paper is fed (pitch feed) by the predetermined amount during continuous batch recording of one line to record an image on the desired range of the recording medium.
In the ink jet type recording apparatus (ink jet recording apparatus), a liquid (ink) is discharged from the discharge element to the recording medium to record an image. Therefore, it is possible to easily make the discharge element compact, rapidly record a high-definition image, and record an image on plain paper without the need for special treatment. This recording apparatus can be run at low cost, causes low noise because of the non-impact system, and has the advantage of easily recording a color image by using many types of inks (for example, color inks).
The ink jet recording apparatus generally comprises driving means provided with a discharge element, for driving a carriage; transfer means for transferring the recording medium (recording paper), and control means for controlling the driving means and the transfer means. On the other hand, as an energy generating element for generating energy used for discharging ink from the nozzle of the discharge element, an element using an electromechanical transducer such as a piezo element, an element for discharging ink droplets by using the exothermic function caused by irradiation with an electromagnetic wave such as laser or the like, or an element for heating a liquid by using an electromechanical transducer having a heating resistor can be used.
Of these elements, the ink jet type discharge element (recording head) for discharging ink droplets by using thermal energy permits high-density arrangement of nozzles, thereby enabling high-resolution recording. Particularly, the discharge element using an electromechanical transducer as the energy generating element can easily be miniaturized, and sufficiently utilize the advantages of IC technology and microprocessing technology which have recently significantly been advanced in the semiconductor field with significant improvement in reliability. Further, high-density packaging can easily be realized, and the production cost is low. Therefore, the discharge element is advantageous.
There are also various requirements for the material of the recording medium. In recent years, development of these requirements has proceeded, and a recording apparatus has been used in which besides paper (including thin paper and processed paper) as a normal recording medium and a thin resin sheet (OHP sheet or the like), fabric, leather, nonwoven fabric, and a metal are used as the recording medium.
A conventional known ink used for the ink jet recording apparatus generally has a composition comprising water as a main component, and a water-soluble high-boiling-point solvent such as glycol or the like which is added for preventing drying and clogging of nozzles. In recording on plain paper by using such an ink, in some cases, the discharged ink is not sufficiently fixed to plain paper, and a nonuniform distribution of a filler or sizing agent on the surface of recording paper possibly causes a nonuniform image. Particularly, in the formation of a color image, inks of various colors are successively discharged to be overlapped at the same position of the recording paper before the inks are fixed, thereby causing a problem of color bleeding in the boundaries between images of different colors, or a problem of forming an unsatisfactory image due to nonuniform mixing of colors.
Therefore, in order to further improve the printing performance of the ink jet recording system, a method is known in which a treatment liquid (or a printability improving liquid) is coated as a liquid for improving the quality of an image on the recording medium before a recording ink is discharged.
For example, Japanese Patent Laid-Open No. 5-202328 discloses a recording method which uses an ink composition comprising at least one chemical dye containing at least one carboxyl group, and a polyvalent metal salt solution. In this method, the polyvalent metal salt solution is applied to the recording medium, and then the ink composition is applied to obtain a good image. Japanese Patent Laid-Open No. 8-193175 discloses an image forming method for obtaining good images, and a printability improving solution and an ink composition, both of which are used in this method.
In the ink jet recording apparatus, as a recovery method for removing clogging of the nozzle of the discharge element, a suction recovery method is used in which the nozzle surface is covered with a cap, and a negative pressure is generated in the cap to suction ink from the nozzle. In order to remove the ink remaining on the nozzle surface after suction recovery of such a discharge element (recording head), a so-called wiping operation is frequently carried out, in which a wiper comprising an elastic material such as rubber or the like is brought into contact with the nozzle surface in order to remove mist of the ink composition which is generated in discharge of the ink during printing and then adheres to the nozzle surface, so that a substance such as ink, mist or dust which adheres to the nozzle surface is removed by relatively moving the wiper to wipe the nozzle surface.
In the system which uses inter-reaction between a treatment solution and an ink composition, as disclosed in the above publication, Japanese Patent Laid-Open No. 10-226058 discloses a discharge element having the construction shown in FIG. 13. FIG. 13 is a plan view of an ink jet discharge element as viewed from the discharge direction of ink.
FIG. 13 shows a plurality of discharge elements for plain paper including black ink discharge elements Bk.sub.1 and Bk.sub.2, a cyan ink discharge element C, a magenta ink discharge element M, a yellow ink discharge element Y, and a discharge element S for discharging a treatment solution for making the dye contained in each of the inks insoluble. In an embodiment of this invention, the discharge elements are scanned in the direction shown by arrow b in FIG. 13 to discharge a printability improving solution from the printability improving solution head (the discharge element for the treatment solution) to recording paper and bring the printability improving solution into contact with the ink discharged from each of the heads (the ink discharge elements) on the recording paper. This provides the dye or the like with water resistance and momentarily makes insoluble the dye in the ink due to reaction with the printability improving solution, thereby sharpening the outline of an image derived from the dye or the like, and preventing bleeding. In recording using the treatment solution, care must be taken to prevent mixing of the treatment solution and ink, and the wiper must be completely separated between at least the treatment solution system and the ink composition system in order to prevent clogging of each of the liquid discharge heads (the discharge elements).
FIG. 14 is a schematic view of a wiping device for wiping the nozzle surface of the discharge elements shown in FIG. 13 as viewed from the movement direction of recording paper. In FIG. 14, the discharge elements (recording heads) 3 are loaded on a carriage 2 which can be scanned in the direction of arrow b, and six blades (wipers) 14 for the respective discharge elements are respectively provided at positions corresponding to the discharge elements of the recording heads 3 positioned at the predetermined positions (wiping positions) in the direction of arrow b. The blades 14 are arranged on a blade holder 15. The blade holder 15 is moved in the direction perpendicular to the drawing to wipe the discharge elements by the blades 14.
However, in the above-described construction, wiping causes the ink or the treatment solution to be pushed toward both sides of each of the wipers, thereby causing a portion remaining unwiped. In this state, recording causes adhesion of a mist and the like to the unwiped portion, and thus causes the phenomenon that a mixture of the ink and the printability improving solution is accumulated on both sides of each of the wiping regions. This finally possibly causes a state in which the accumulated mixture drops on the recording paper or is transferred thereto and thus stains the recorded image, thereby deteriorating image quality. An example of the mechanism of such a phenomenon is described with reference to FIGS. 15 and 16. FIG. 15 is a plan view showing the discharge elements in a state after wiping as viewed from the discharge direction of ink, and FIG. 16 is a schematic drawing showing ink droplets in a state after printing as viewed from the movement direction of recording paper.
In wiping for removing the ink remaining on the nozzle surface of each of the discharge elements after suction recovery, and mist of the ink composition or the like which is generated in discharge and adheres to the nozzle surface after discharge, the nozzle surface is put into a state in which the wiping region shadowed in the drawing which contacts each of the blades 14 is cleaned, and the removed ink is forced or excluded to both sides of the wiping region, as shown in FIG. 15. Therefore, stripes of the removed ink occur on the nozzle surface. In this state, a printing operation causes scattering of main droplets V, floating mist comprising small droplets, which are discharged from the nozzle N but do not reach the recording paper P, and the presence of mist R rebounding upon recording paper P between the discharge elements 3 and the recording paper P. These substances adhere to the nozzle surfaces of the discharge elements 3 with the operation of scanning the carriage 2.
At this time, when the mist adheres to the removed ink stripes on both sides of the wiping region, the mixture of the ink and the printability improving solution gradually grows. For example, when the mist of the printability improving solution discharged from the discharge element S adheres to the removed ink accumulations on the surface of the discharge element Bk.sub.2, the dye in black ink reacts with the printability improving solution to be made insoluble. Then, the excluded ink Bk is accumulated by the next wiping operation, and the mist of the printability improving solution again adheres to the accumulated ink Bk to produce a reaction product. This is repeated to accumulate the mixture of the ink accumulated on both sides of the wiping region and the printability improving solution, and cause the growth thereof. The accumulated mixture finally contacts the recording paper during the operation of scanning the carriage 2, thereby staining an image.